1. Field of the Invention
This invention relates to optical measurement apparatus and has application in the measurement of displacement, for example in pressure sensors in which pressure is indicated by the displacement of a diaphragm.
2. Description of Related Art
One known arrangement for measuring displacement comprises an optical fibre the tip of which is positioned to direct light transmitted along the fibre onto a reflection element or a transmission element the displacement of which is to be measured. A reflection element may comprise a diaphragm or membrane having a reflecting surface and forming part of a pressurised enclosure. The optical system is arranged so that the change in the amount of light reflected or transmitted is a measure of the displacement of the element. The reflected or transmitted light is measured and its value indicated to give a measure of displacement or pressure. Desirably the optical system is arranged so that the change in reflected or transmitted light is linearly related to displacement.
While a sensor of the kind described above is accurate immediately after calibration it will be appreciated that the magnitude of the detected signal depends not only on the position of the element but also on any change in the quality of the optical path that occurs after calibration. The path and its constituents may be subject to ageing which will affect the accuracy of the measurement.
It is an object of the invention to provide compensation for the effects of such change in quality.